Cell culturing is an essential step in manufacturing biological products such as, for example, nucleic acids, viruses for use in vaccines, antibodies, and proteins, for example, interferons. Anchorage-dependent cells, such as certain animal cells, need to attach to a surface in order to grow and divide.
For large-scale cell culturing, microcarriers provide the large surface area needed for growing anchorage-dependent cells. Van Wezel, in 1967, described the use of microcarriers, small beads or particles approximately 0.2 mm in diameter, for growing such cells. Using gentle agitation, the microcarriers to which the cells will attach are suspended in a liquid culture medium within a bioreactor.
The process may begin with the addition of cells (the inoculum) to the liquid culture medium in which the microcarriers are suspended. The culture medium contains the nutrients essential for metabolism and growth of the cells. Conditions of temperature, pH, and oxygen concentration are controlled to promote cell growth and division in order to increase cell density and confluence.
Continuous or Perfusion Mode: In a continuous or perfusion mode, nutrients are continuously added to the system, and product is harvested throughout the culture period. With the continuous mode, the on-going difficulty in obtaining sufficiently high product titers is well recognized. In addition to the low titer issue, there is a need to concentrate product of the continuous mode. These problems have a direct impact on production time and cost, and make the continuous mode less feasible at least for vaccine production.
Batch Mode: In a batch mode, all nutrients are added at the beginning and products are not removed until the end of the batch. Waste products accumulate during the run, and nutrients are used up, making the batch process inefficient for many applications.
Fed-Batch Mode: A fed-batch mode is similar to the batch mode in that products are removed only at the end of the run, but differs in that nutrients are added at multiple intervals during the process. Most virus-producing, microcarrier cultures are carried out, post infection, in a fed-batch process. In the fed-batch mode, there is also an increase in waste products and other contaminants, such as host cell protein and host cell DNA, and dead cells falling off of the microcarriers.
Thus, there remains an on-going need for a new process that increases product titer while eliminating the problems inherent in the batch mode and the fed-batch mode of cell culturing. Moreover, techniques that can increase the yield, production efficiency or speed of harvesting viral products for vaccines, in particular, would also satisfy an on-going need and permit the health care system to respond more rapidly to new viral outbreaks.